Aircraft main engines not only provide propulsion for the aircraft, but in many instances are used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main engines may not be capable of supplying power. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may additionally be used to start the main propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustor, at least one power turbine, and a compressor. During operation of the APU, the compressor draws in ambient air, compresses it, and supplies compressed air to the combustor. The combustor receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy compressed air to the power turbine, causing it to rotate.
Many APUs include multi-stage turbines with each generating work to drive other components such as a generator and a compressor impeller. The first-stage turbine is the first to receive high energy compressed air from the combustor, and is consequently subjected to temperatures of up to 1960° F. (1071° C.). The second-stage turbine receives the air after it flows past the first stage turbine blades. The air is substantially cooler when it reaches the second-stage turbine.
As the first and second-stage wheels are subjected to different operating temperatures, they are manufactured to have different structural and metallurgical properties. Many conventional first-stage turbines include an inner disk and individually cast blades that have machined fir tree or dovetail attachments that enable blade insertion into mating machined slots in the rim of the disk. The inserted blade design enables each of the blades to be coated with materials that can be applied using an overlay process and that are typically more resistant to a hot and corrosive environment than a diffusion bond coating, and to perform the coating methods before assembling the blades on the disk. The inserted blade design also enables the use of a disk material that is different from that of the blades to provide long term durability and low cycle fatigue (LCF) life for the first-stage turbine.
First-stage turbines having the inserted blade design may experience axial blade shift or blade walk during engine operation. Although it is desirable to eliminate the potential for axial shift or walk, to date there has not been a suitable alternative to the inserted blade design that bestows suitable metallurgical properties for the disk and blades. Also, the high operational temperatures and attachment stresses that are subjected on the turbines require machining the individual blades and slots to tight tolerances. This involves excessive labor and time. Accordingly, it is desirable to provide a first-stage APU turbine that is not susceptible to axial blade shift or blade walk, but that is also capable of operating at very high temperatures and in a highly corrosive environment. In addition, it is desirable to provide a first-stage APU turbine that includes a disk and blades with different metallurgical properties. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.